False signal prevention system including mechanical member physically displaceable by vibration and shock



' HLHLHENUL Shhfiiiii H'Ui J. E. TREATCH 3,4 6, 35 FALSE SIGNAL PREVENTION SYSTEM INCLUDING MECHANICAL MEMBER PHYSICALLY DISPLACEABLE BY VIBRATION AND SHOCK Filed Sept. 9. 1965 Dec. 10, 1968 w T mm mm A m R P Om ET 1 5&3 M E 522 a 2T m 8 mm J 8 ON H A Y 553 58 mm B W R @9850 E2 52% 23 M19 5mm wmmm 6. 5 :1 1 85 528mm 052 525% N 8 F. E

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United States Patent 3,416,135 FALSE SIGNAL PREVENTION SYSTEM INCLUD- ING MECHANICAL MEMBER PHYSICALLY DIS- PLACEABLE BY VIBRATION AND SHOCK James E. Treatch, Chicago, Ill., assignor to Motorola, Inc., Franklin Park, 11]., a corporation of Illinois Filed Sept. 9, 1965, Ser. No. 486,165 5 Claims. (Cl. 340-156) ABSTRACT OF THE DISCLOSURE System for inhibiting false operation of electronic equipment having mechanically resonant frequency responsive device with false output being inhibited by mechanical device having resilient portion which is physically displaced in response to mechanical vibrations and shock. The mechanical device includes electro-mechanical unit for producing an inhibit signal which is applied to a circuit which inhibits the output of the electrical equipment. An inhibit circuit including a transistor responds to the signal to disable the output of the electrical equipment.

This invention relates to selective paging or calling systems, and particularly to such systems wherein the receivers include mechanical devices resonant at different frequencies for address purposes and in which false calls resulting from accidental mechanical jarring of the resonant device are prevented.

One system representative of the present selective paging systems employs a central call station transmitter and a number of receivers tuned to the carrier frequency of the call station transmitter. Each receiver is assigned a call signal of certain audio frequency or combination of audio frequencies, and its decoding circuits incorporate a reed or reeds sensitive to that frequency or frequencies. The central station calls a receiving station by transmitting a carrier frequency modulated by the audio call signal unique to that receiver. The carrier frequency is received by all stations, but only that station with a reed sensitive to the call signal frequency responds by turning on an audio oscillator for annunciation. However, because these remote receivers are usually carried on the. person, they are subject to considerable jarring, bumping, and impact, which can sometimes cause the reed to vibrate without electric excitation, thus producing a false call.

It is an object of this invention to provide a selective paging or calling system which prevents false calls resulting from mechanical shock, and which is extremely simple in principle and application.

It is a further object of this invention to provide such a system which is automatically reset for further reception as soon as the unwanted mechanical shock has subsided.

It is another object of this invention to provide a system for inhibiting operation of a paging receiver in response to mechanical shock which can be added to existing paging systems without making it an integral part of the design of the system, so that retrofitting existing systems can be accomplished easily.

A feature of this invention is the provision of paging system which includes mechanical apparatus responsive to shock for producing a signal for inhibiting the operation of a selective system. The inhibiting apparatus can be made very small without impairing the adequacy of the electric signal obtained from it.

Another feature of this invention is the provision of a spring mounted mass which operates in response to a mechanical shock to produce an inhibit signal which can be applied to various points in a typical paging receiver to inhibit the output signal. The mechanical inhibit ap- 3,416,135 Patented Dec. 10, 1968 paratus can be added to existing circuits without redesign and the components required are few and inexpensive.

A further feature of this invention is the provision of a mechanical inhibit device which is directly responsive to mechanical shock which can cause false operation of the mechanically resonant device of a paging system, and which closes a switch to disable the paging system so that false calls are prevented without the use of elaborate coding.

The invention is illustrated in the drawings wherein the single figure is a schematic diagram illustrating the inhibit device applied to the receiver of a paging system.

The invention provides a movable mechanical mass which is suspended near a stationary metal surface in such a way as to touch the surface only upon shock or impact. The mass is supported by a spring which acts to return the mass to its normal position after impact. The mass and surface are electrically conductive and are connected to the input of an amplifier or oscillator circuit in a way to short one circuit element to another, or to apply a voltage to an element, thus biasing off that stage when a circuit closure is present because the mass contacts the surface. An intermediate transistor can be used in a switching mode to connect the mechanical device to the circuit. In this use the closure between the mass and the surface places a forward bias across the base and emitter of the transistor, turning it on and effectively shorting out the circuit between the collector and emitter by the resulting low impedance. An RC circuit section can be added to the circuit to provide a time delay'after the closure, during which the transistor remains in the on condition.

FIGURE 1 shows a frequency modulation radio paging receiver system utilizing the inhibit device of the invention. Antenna 11 is connected to the first stages of a frequency modulation receiver 12, with the output of the receiver discriminator stage 13 being connected to a reed driver stage 14. A reed device 15 includes two coils, magnetically coupled to a single tine resonant electromechanical device. When the input coil is driven by an electrical oscillatory current of the same frequency as the resonant tine, the tine vibrates strongly, magnetically coupling the input coil to the out-put coil which is connected to the tone amplifier 16. The tone detector 17 responds to this amplified tone.

The paging system includes an oscillator having a transistor 25. This oscillator is of the Clapp type with inductor 21 and capacitors 20, 22 and 23 forming the frequency determining circuit. The base-emitter bias for the transistor 25 is developed across resistor 19. Power supply 26 applies a negative direct current potential (--8 volts) on line 38 to the emitter, with resistor 24 providing dc stabilization.

The tone detector 17 in the standby condition applies a potential to resistor 18 which is substantially the same as the potential (-8 volts) applied by the power supply on line 38 to resistor 19. Accordingly, there is no substantial potential across resistor 19 and transistor 25 will not conduct. When the tone is applied to detector 17, the potential applied to resistor 18 falls to a low value (-0.6 volt) so that a potential is developed across resistors 18 and 19. These resistors act as a divider to apply a potential to the base of transistor 25 which is positive with respect to the potential at its emitter. This potential forward biases transistor 25 so that it conducts.

When the oscillator transistor 25 is forward biased by the output of the tone detector 17, oscillations begin. These oscillations are coupled through a selector switch 27 into the audio amplifier section 28, producing an audible page call from the speaker 29. When the selector 27 is turned from page mode to receiver mode, this output from the oscillator is disconnected, and the output of the discriminator stage 13 is connected directly to the audio section 28 for the message.

A preferred form of the invention includes a small metal tube 33 supported on an insulator 34 and connected to the line 39 from the power supply 26 which which applies a negative potential (-5 volts). A spring arm 31 and a weight 30, both of which are electrically conductive, are supported along the axis of the tube 33 by an insulating plug 32 which fits the end of the tube securely and into which is inserted the spring arm 31. The plug thus serves the dual purpose of positioning the weight and arm in the tube and of insulating them for the tube. A transistor 37 of the NPN type has its emitter connected to the (-8 volt) line 38 from the power supply and its collector connected to the base of the oscillator transistor 25. The base of transistor 37 is connected through a current limiting resistor 36 to the spring arm 30. A capacitor 35 is connected between the spring arm 31 and the line 38.

Should the receiver be subjected to shock that causes the reed of device to vibrate to produce a voltage in the output coil, the output signal of the reed device would be amplified and detected, and cause the oscillator to turn on. However, the same shock. would cause the weight 30 to move into contact with the tube 33. This would forward bias transistor 37, turning it on strong which in effect would short the transistor 25 base to the line 38. This would cause removal of the forward bias developed across resistor 19, thus turning ofi the oscillator 25. Capacitor 35 which is now across the potential between lines 38 and 39 charges to the voltage dilference, and when the weight 30 moves away from the tube 33, the voltage on capacitor 35 keeps the transistor 37 turned on and the oscillator inhibited. The RC circuit formed by resistor 36, capacitor 35 and base-emitter junction of the transistor 37 forms a time delay dependent upon the values of resistor 36 and capacitor 35.

The small size of the contact arran-gement renders it suitable for use in small devices such as pocket size paging receivers. The tube 33 can be less than A; inch in diameter and approximately one inch long.

It will be immediately apparent that it would be possible to apply this protective circuit to almost any of the active stages in this receiver and obtain the same inhibiting efiect on the final speaker output. The circuit can also be easily added to existing equipment.

The advantage of using a transistor 37 in a switching mode instead of applying the circuit closure directly is to place a buffer between the oscillator circuit and the inherent capacitance of the device as described. A simple circuit change would make it possible to reverse the action of transistor 37, using a circuit opening structure for the mass and tube.

The protective device described has been found to be highly effective in use. Inasmuch as the device is small and inexpensive it is suitable for use in many applications.

I claim:

1. in electrical equipment adapted to respond to a calling signal of a given frequency and which includes a frequency discriminator having a mechanically resonant device responsive to the given frequency and electrically excited by the incoming calling signal, and which is subject to being excited by external mechanical vibrations and shocks to produce a false output, apparatus to prevent a false output from the equipment in response to external mechanical vibrations including in combination, mechanical means supported on the electrical equipment and having a resilient portion which is physically displaced in response to physical movement of the electrical equipment, said mechanical means including electro-mechanical means for producing an inhibit signal in response to physical displacement of said resilient portion, and circuit means coupled to the electrical equipment and responsive to said inhibit signal to inhibit the output of the electrical equipment.

2. Apparatus in accordance with claim 1 wherein said mechanical means includes a mass supported by said resilient portion.

3. Apparatus in accordance with claim 1 wherein said electro-mechanical means includes a switch having a fixed contact and a movable contact coupled to said resilient portion and moved into engagement with said fixed contact by displacement of said resilient portion.

4. Apparatus in accordance with claim 3 wherein said movable contact has substantial mass and said fixed contact is a tubular conductor positioned about said movable contact.

5. Apparatus in accordance with claim 1 wherein said circuit means includes a transistor coupled to the electrical equipment and responsive to said inhibit signal to inhibit the output of the electrical equipment.

References Cited UNITED STATES PATENTS 2,122,921 7/ 1938 Sands ZOO-61.47 2,161,411 6/1939 Erich ZOO-61.47 2,195,417 4/1940 Mason 317-144 2,983,902 5/1961 Philipps 340-156 3,109,165 10/1963 Bagno 340-258 3,232,119 2/ 1966 Salerno ZOO-61.47 X 3,109,165 10/1963 Bagno 340-258 2,778,896 1/ 1957 Tollefsen 200-61.4S 3,039,081 6/ 1962 Smith 340-171 3,160,868 12/1964 Kowanda 200-61.45 X 3,284,791 11/1966 Voight et al 340-171 X JOHN W. CALDWELL, Primary Examiner.

H. PIT'IS, Assistant Examiner US. Cl. X.R.

340-171; 331-62; 325-473; ZOO-61.45 

